1. Field of the Invention
This invention relates to a one or two-step process of treating high-sulfur caking coals for desulfurization, elimination of caking tendencies and incorporation of a gasification catalyst therewith.
2. Description of the Prior Art
The United States has an ample supply of coal to meet energy demands for several centuries, and many processes have been proposed for utilizing this enormous resource. In some, the coal is partially gasified in steam and/or oxygen (1-5) or hydrogen (6,7) to produce combustible gases, while others use the coal primarily as a boiler fuel. A large portion of the available coal, however, is not amenable to direct utilization because it is high in sulfur or caking, or frequently, both.
High-sulfur coals present a serious problem because Federal regulations impose restrictions on the amounts of SO.sub.x that can be emitted per M BTU of energy value. Thus fuel gases must be cleaned of H.sub.2 S before combustion, or the stack gases must be scrubbed of SO.sub.x after combustion, while the combustion gases from chars or coals used as boiler fuels must also be scrubbed, or the coals must be cleaned prior to use.
Coal may be cleaned by either mechanical or chemical means. In mechanical cleaning of coal, the coal is crushed, sieved, and roughly sorted in largely inorganic pyrite (FeS.sub.2) and carbonaceous matter by physical properties. This removes roughly half of the sulfur (the inorganic pyrite), along with a considerable loss in BTU values because of poor mechanical spearation. Chemical means, such as the Meyer's Process (8), chemically oxidize the inorganic sulfur to sulfate and free sulfur: EQU FeS.sub.2 +4.6Fe.sub.2 (SO.sub.4).sub.3 +4.8H.sub.2 O-- -- -- FeSO.sub.4 +4.8H.sub.2 SO.sub.4 +0.8S EQU 2.4O.sub.2 +9.6FeSO.sub.4 +4.8H.sub.2 SO.sub.4 -- -- -- 4.8Fe.sub.2 (SO.sub.4).sub.3 +4.8H.sub.2 O
Caking coals present an altogether different type of problem than do high-sulfur levels. Under mild heating, caking coals clump, stick together, or agglomerate into a mass that is difficult to move, handle, or process. Thus, most processes partially oxidize caking coals to provide a non-sticking exterior prior to gasification, even if the coal is subsequently to be used in a hydrogasifier where hydrogen consumption is an important process variable.
Gasification processes have been found to be more favorable if inorganic promotors, or catalysts, are added to the coal. These include such materials as alkali metal carbonates, lime, iron salts, or even coal ash itself. Generally, they are dissolved or slurried in water and mixed with the coal and dried before gasification.